Imagers employ either a two-dimensional (2D) multichannel detector array or a single element detector. Imagers using a 2D detector array measure the intensity distribution of all spatial resolution elements simultaneously during the entire period of data acquisition. Imagers using a single detector require that the individual spatial resolution elements be measured consecutively via a raster scan so that each one is observed for a small fraction of the period of data acquisition. Prior art imagers using a plurality of detectors at the image plane can exhibit serious signal-to-noise ratio problems. Prior art imagers using a single element detector can exhibit more serious signal-to-noise ratio problems. Signal-to-noise ratio problems limit the utility of imagers applied to chemical imaging applications where subtle differences between a sample's constituents become important.
Spectrometers are commonly used to analyze the chemical composition of samples by determining the absorption or attenuation of certain wavelengths of electromagnetic radiation by the sample or samples. Because it is typically necessary to analyze the absorption characteristics of more than one wavelength of radiation to identify a compound, and because each wavelength must be separately detected to distinguish the wavelengths, prior art spectrometers utilize a plurality of detectors, have a moving grating, or use a set of filter elements. However, the use of a plurality of detectors or the use of a macro moving grating has signal-to-noise limitations. The signal-to-noise ratio largely dictates the ability of the spectrometer to analyze with accuracy all of the constituents of a sample, especially when some of the constituents of the sample account for an extremely small proportion of the sample. There is, therefore, a need for imagers and spectrometers with improved signal-to-noise ratios.
Prior art variable band pass filter spectrometers, variable band reject filter spectrometers, variable multiple band pass filter spectrometers or variable multiple band reject filter spectrometers typically employ a multitude of filters that require macro moving parts or other physical manipulation in order to switch between individual filter elements or sets of filter elements for each measurement. Each filter element employed can be very expensive, difficult to manufacture and all are permanently set at the time of manufacture in the wavelengths (bands) of radiation that they pass or reject. Physical human handling of the filter elements can damage them and it is time consuming to change filter elements. There is, therefore, a need for variable band pass filter spectrometers, variable band reject filter spectrometers, variable multiple band pass filter spectrometers or variable multiple band reject filter spectrometers without a requirement for discrete (individual) filter elements that have permanently set band pass or band reject properties. There is also a need for variable band pass filter spectrometers, variable band reject filter spectrometers, variable multiple band pass filter spectrometers or variable multiple band reject filter spectrometers to be able to change the filters corresponding to the bands of radiation that are passed or rejected rapidly, without macro moving parts and without human interaction.
In several practical applications it is required that an object be irradiated with radiation having particularly shaped spectrum. In the simplest case when only a few spectrum lines (or bands) are necessary, one can use a combination of corresponding sources, each centered near a required spectrum band. Clearly, however, this approach does not work in a more general case, and therefore it is desirable to have a controllable radiation source capable of providing arbitrary spectrum shapes and intensities. Several types of prior art devices are known that are capable of providing controllable radiation. Earlier prior art devices primarily relied upon various “masking” techniques, such as electronically alterable masks interposed in the optical pathway between a light source and a detector. More recent prior art devices use a combination of two or more light-emitting diodes (LEDs) as radiation sources. In such cases, an array of LEDs or light-emitting lasers is configured for activation using a particular encoding pattern, and can be used as a controllable light source. A disadvantage of these systems is that they rely on an array of different LED elements (or lasers), each operating in a different, relatively narrow spectrum band. In addition, there are technological problems associated with having an array of discrete radiation elements with different characteristics. Accordingly, there is a need for a controllable radiation source, where virtually arbitrary spectrum shape and characteristics can be designed, and where disadvantages associated with the prior art are obviated. Further, it is desirable not only to shape the spectrum of the radiation source, but also encode its components differently, which feature can be used to readily perform several signal processing functions useful in a number of practical applications. The phrase “a spectrum shape” in this disclosure refers not to a mathematical abstraction but rather to configurable spectrum shapes having range(s) and resolution necessarily limited by practical considerations.
In addition to the signal-to-noise issues discussed above, one can consider the tradeoff between signal-to-noise and, for example, one or more of the following resources: system cost, time to measure a scene, and inter-pixel calibration. Thus, in certain prior art systems, a single sensor system may cost less to produce, but will take longer to fully measure an object under study. In prior art multi-sensor systems, one often encounters a problem in which the different sensor elements have different response characteristics, and it is necessary to add components to the system to calibrate for this. It is desirable to have a system with which one gains the. lower-cost, better signal-to-noise, and automatic inter-pixel calibration advantages of a single-sensor system while not suffering all of the time loss usually associated with using single sensors.
In a conventional monochromater, white light or broadband optical energy emerging from an entrance slit is collimated onto a diffraction grating, angularly dispersed according to wavelength and then focused onto an exit slit. In this way, relatively monochromatic light or optical energy confined to some narrow band defined by the geometry and physical properties of optical elements and their arrangement emerges from the exit slit. By moving or translation of one or more of the slits left and right or rotating the grating while leaving the slits stationary, the wavelength of the monochromatic output light scans through the wavelength range of the device. The resulting output sequence interacts with the matter, and the results of each are measured. Alternatively, the light interacts with the matter prior to passing through the spectrometer and the resulting output sequence is then measured. In either case, the resulting sequence of measurements gives a spectral signature of the matter. However, such devices are deficient in that the two slits required to help define the bandpass located both at the object and image plane of the optical system work to severely limit the amount of light that passes through the system, and therefore limit the amount of light that is measured. For this reason, it is difficult or impossible to rapidly obtain good signal to noise ratios with monochromaters where high resolution is desired or in situations where there is a limit to the amount of light energy available.
Conventional linear array spectrometers can be viewed as an improvement over the monochromaters. In a conventional linear array spectrometer, the output slit located at the image plane or focal plane of the optical system is replaced by a linear array of detectors situated such that there is a detector at each exit slit position to receive the light. For such spectrometers, light interacts with the matter prior to passing into the monochromater, and the linear array of detectors simultaneously measures the resulting sequence of spectral data. The intensity of the set of bands of wavelength that impinge upon the linear array of detectors during the integration time of the measurement provides the spectral signature of the matter. Linear array spectrometers have an advantage over monochromaters in that the linear detector array collects all of the data simultaneously such that fluctuations in the source energy are not interpreted as features of the spectral signature of the matter. Additionally, unlike the monochrometers, the linear array spectrometers have no moving parts and can make instantaneous measurements. Further, during the time it takes to collect a spectrum using a scanning type monochrometer, the linear array spectrometers can collect multiple spectra. Conversely, the linear array spectrometer can collect an entire spectrum in the time it takes the scanning monochrometer to collect one spectral resolution element. However, the entrance slit still limits the amount light entering the system to each detector element in the linear detector array.
Conventional Hadamard spectrometers can be viewed as an improvement over both the scanning monochromaters and linear array spectrometers. In conventional Hadamard spectrometers, one or both of the slits of a monochromater are replaced by a coded array of slits (or mask). Thus, the exit light is no longer monochromatic in nature, but is an encoded mixture of wavelengths of light where the encodement is determined by the optical masks that can be located at the object or image planes of the optical system. The conventional Hadamard spectrometer operates by changing or moving one or both of the mask(s) through a pre-determined sequence of changes or moves. In this way, a full encodement library of exit light is produced. The light entering or exiting the optical system interacts with the sample or matter, and the results of each of the encodements are measured. The measurements of the light resulting from the interaction with the sample or matter and sequence of encoding combinations dictated by mask positions or encodements, is then mathematically inverted, so that one reconstructs the spectral signature of the sample or matter. Since the Hadamard spectrometer has many more slits than the monchrometer, more light is available at the exit aperture. However, the conventional Hadamard spectrometers have changing or moving parts to move or translate the encoded aperture through the requisite combinations of encodements to be measured. Such motion or change due to physical limitation of the conventional Hadamard spectrometer is generally subject to some variation, error and/or distortion over time, and is especially susceptible to errors in the presence of noise, heat, and other environmental or mechanical disturbances.
As noted herein, conventional spectral measurement systems, such as the scanning monochromaters suffer from these attributes noted herein. The linear array spectrometers and scanning monochromaters suffer from a lack of light throughput. Conventional spectral measurement systems, such as the monochromaters, Hadamard spectrometers and Fourier transform spectrometers suffer from a complexity and instability due to the presence of moving or changing parts. Since the latter spectral measurement systems make a series of measurements over time, rather than instantaneously, each suffers from errors when it is looking at light sources or sample/matter that is changing during the time of measure or scan. A further disadvantage of the scanning monochrometers, linear array spectrometers and Fourier transform spectrometer systems is that a contiguous regular interval of wavelength spectral data are collected. In many spectrometric applications, such contiguous spectral data generally contains no relevant or useful information with respect to the spectral signature of the sample or matter. Hence, it is desirable to collect non-contiguous variable band pass spectral resolution element data that comprise only those spectral bands that are deemed of significance to the spectrometric measure or analysis of the desired sample or matter.
Accordingly, there is a need for a spectral measurement system that offers the advantages of both the linear array spectrometers and Hadamard or Fourier spectrometers. Additionally, there is a need for a spectral measurement system capable of collecting only the non-contiguous and non-uniform band pass spectral data necessary for the desired analysis. The spectral measurement system of the present invention comprises multi-detector and no moving parts and provides instantaneous measurements.
In prior-art dispersive systems, as one moves to high resolution spectrometric measures one must decrease the slit width when preserving spectral bandwidth (spectral range) or increase the angular dispersion of the diffraction element and this results in a decrease of spectral range. Both the decrease in slit width and increase in angular dispersion decrease the photon flux through the spectrometer for any given measure of individual spectral resolution elements. This decrease in photonic flux limits sensitivity, analytical capability and the ability of the detector to measure subtle differences in spectral energy at each data resolution element or related spectral resolution element. Hadamard transform measurements allow for multiplexing in which a multitude of slits are opened simultaneously for each measurement. Fourier transform measurements allow for multiplexing in which there is no slit in the system, and each wavelength is measured with a Fourier weight that varies over time. The advantage of these multiplexing techniques is that photonic flux improves over single slit methods as the demand for resolution or data resolution elements increase for a given spectral bandwidth of operation. The drawback to increasing the spectral resolution requirement that can apply to these conventional methods, even Hadamard and Fourier Transform multiplexed measurements, is that the scan times may need to be increased so that the integration times for each of the data resolution elements in the scan is sufficient to collect enough signal to rise above the noise floor of the system and to achieve the desired measure of the requisite signal-to-noise. The conventional multiplexing methods require measuring a series of encodements over time T which dictates a maximum detector integration time of T/N where N is the number of resolution elements. This requires an increase in scan time and can be problematic if the source energy or sample changes over the time T of the scan. The spectral measurement system of the present invention based on multiplexing spectrometry proceeds upon the desirability of eliminating such problem as each detector views the source or sample fluctuations simultaneously.
In the spectral measurement system of the present invention, the scan is not intermodulated by these changes and the intermodulations do not show up as noise in the spectral data. This has been the impetus for fast scan type Fourier Transform (FT) spectrometer systems. Despite the speed increase in scan times, these fast scan systems are inadequate for many measures such as looking at spark or combustion products during the life of a burn cycle. The spectral measurement system of the present invention based on static spectrometer is capable of looking at rapidly changing sources, samples and environments while taking advantage of the high throughput afforded by multiplexed measurements.
Many point spectrometers whether multiplexed or not utilize single detectors. Historically this has in large part been due to the lack of availability, reliability, or affordability of appropriate detector arrays. Most applications that did not overtly require linear or two-dimensional arrays were hastily assigned to single detector solutions. However, detector arrays, when properly employed, can enable advantages such as faster collection rates, oversampling, and better signal to noise. One of the first to realize this was S. Mende in “Hadamard spectroscopy with a two-dimensional detecting array”, S. B. Mende; E. S. Claflin; R. L. Rairden; G. R. Swenson, Applied Optics 32 (34): 7095–7105 (Dec. 1, 1993), in which he made the clever observation that for diffuse sources one could illuminate a two-dimensional coded aperture spectrograph and to obtain accurate and high resolution measurements. Each row of the mask consisted of a Hadamard sequence parallel to the direction of dispersion. The dispersed output from that row was recorded across the corresponding row of a two-dimensional focal plane array. Using that data, he proposed a scheme to invert the data and reconstruct the spectrum of the uniform light illuminating that row. Different Hadamard sequences of varying lengths were put on each row providing redundant information for averaging, or, if the source were uniformly illuminating across rows but spatially coherent across columns, 1-D spectral imaging could be obtained. In either case, the measurement required no mechanical scanning and only one frame of acquisition. His invention was limited by the numerical conditioning of the mathematical inverse required to reconstruct the spectra from the data collected. He also neglected to consider the possibilities of a linear array in similar configurations.
The present invention differs from Mende's in several ways. Each row of the coded aperture contains Hadamard basis vector generated by the same Hadamard sequence. In one embodiment, this would mean that each row would contain a cyclic shift of a particular Hadamard sequence. Dispersion is still parallel to the rows, but the detector no longer needs to measure across the corresponding row to compute the spectra of the impinging light but rather across the columns. This removes the mathematical issues of Mende's approach and also no longer requires a two-dimensional array which can still difficult to afford or obtain for many wavelength ranges. Furthermore with a linear array perpendicular to the rows, acquisition can be even faster. A two-dimensional array can still be employed in the current invention, yielding redundant information to improve SNR for example by averaging.
It should be pointed out that in the subsequent U.S. Pat. No. 5,627,639, Mende discloses various coded aperture approaches to spectral imaging, and does consider one configuration in which each row is a generated by shifts of the same sequence, as in some embodiments of the present invention. However, he only discloses this in the context of spectral imaging and with the added requirement that the mask (or scene) be scanned. Again, the present invention is for diffuse input only and does not resolve spatial information, i.e., image. It requires no scanning and yields an instantaneous measurement that Mende's scanning method does not. Furthermore, other embodiments of the present invention are not designed to recover the full spectrum but rather “filters” or weighted combinations of wavelengths derived from simple or sophisticated mathematical and chemometric models. These measurements rapidly yield quantities of interest at reduced data rates. None of Mende's disclosures have any provision for this; the full spectrum is always being measured.
While there are more and more intelligent spectral devices being developed that are capable of measuring quantities of interest as opposed to a full raw spectrum which is analyzed post acquisition, most either involve some kind of clever active illumination or scanning filters and or a mechanically moving part—in the best case a MEMS device.